Semiconductor devices are commercially well established as active components in computing and communication equipment where they may serve, e.g., as gates and switches and as sources, modulators, and amplifiers of radio frequency electromagnetic radiation. Semiconductor devices are being developed also for serving similar functions in optical communications as exemplified by devices such as laser diodes and phototransistors described in the book by A. A. Bergh et al., Light Emitting Diodes, Clarendon Press, 1976.
Among semiconductor devices for radio frequency application are devices generally known as metal-semiconductor field effect transistors (MESFETs for short) which are being used, e.g., as microwave amplifiers on account of their capability to operate at relatively high frequencies and power levels and to generate low noise output. A basic MESFET structure is described in the book by S. M. Sze, Physics of Semiconductor Devices, Wiley-Interscience, 1969 where, on p. 411, a device is shown to encompass a semi-insulating substrate on which an n-type semiconducting epitaxial layer is deposited. Three contacts are deposited on the semiconducting layer, namely a first ohmic contact acting as a negative or grounded source contact, a metal-semiconductor Schottky barrier contact acting as a gate contact, and a second ohmic contact acting as a positive drain contact. A device structure is disclosed also on p. 211 of the paper by J. V. DiLorenzo et al., "GaAs+FET=Improved Microwave Systems", Bell Laboratories Record, September 1978, pp. 209-215. Devices disclosed by DiLorenzo exhibit a gate electrode which is disposed in a notched portion of the active layer.
Design of MESFETs is aimed at the realization of a number of desirable device features such as, e.g., maximization of gain and output power. In this respect, various impediments have been described in the literature among which are, e.g., effects known as source-drain burnout and gate breakdown. Burnout, in particular, is addressed by M. Fukuta et al., "Power GaAs MESFET with a High Drain-Source Breakdown Voltage", IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-24, No. 6, June 1976, pp. 312-317 and by S. H. Wemple et al., "Source-Drain Burn-Out in GaAs MESFETs", Inst. Phys. Conf., Ser. No. 33B, 1977, pp. 262-270. A device design for minimizing burnout is disclosed in pending U.S. patent application Ser. No. 921,585, filed July 3, 1978, in the names of W. C. Niehaus and S. H. Wemple.
Gate breakdown, also known as gate-drain avalanche, is disclosed, e.g., in the paper by M. Fukuta et al. cited above; the paper by M. Fukuta et al., "GaAs Microwave Power FET", IEEE Transactions on Electron Devices, Vol. ED-23, No. 4, April 1976, pp. 388-394; and by J. V. DiLorenzo et al., IEEE Transactions, Vol. MTT-27 (1979). Gate breakdown occurs at a characteristic gate-drain voltage V.sub.gd and is manifested by excess drain current which cannot be modulated by the gate. Since, in the interest of gain and output power capability of devices, elevated gate-drain operating voltages are desirable, means are sought to optimize breakdown voltage V.sub.gd.